Study On Fe Isotope Fractionation During Magmatic Differentiation And Experimental Constraints On Cu Isotope Fractionation Between Magma And Hydrothermal Fluid

With the advent and development of the analytical technique based on MC-ICP-MS,precise analyses of isotope compositions of heavy atoms become practicable.High-precision of the isotope data allows resolution of the small isotope fractionation at high temperature.To data,extensive measurement of isotope compositions for both terrestrial and extra-terrestrial samples have been conducted.Better understanding of the isotope dataset and greater extent of application of non-traditional stable isotopes in geological processes require the knowledge of isotope fractionation mechanism.Theoretical calculation,measurement of natural samples,and experimental determination have been used to study the mechanism of isotope fractionation.In this thesis,we focus on Fe and Cu isotope systematics.With respect to the geochemical characteristics and research gaps in isotope fractionation mechanism of both isotopes,(1)we investigated the mechanism of Fe isotope fractionation in a suite of well-characterized volcanic rocks,and(2)we experimentally calibrated the Cu isotope fractionation between magmatic-hydrothermal fluid and silicate magma.This study presents Fe-Zn isotope data for a suite of well-characterized bimodal volcanic rocks from Hailar Basin,northeast China to understand the mechanism of Fe isotope fractionation in highly differentiated igneous rocks.The samples range from basaltic trachyandesitCs to trachytes-rhyodacites,and rhyolites.The ?56Fe values increase with increasing SiO2 contents with the rhyolites having the highest ?56Fe(up to 0.64±0.02‰)among the previously reported data for igneous rocks at a similar SiO2.The lack of correlation between ?56Fe and Rb/La argues against the effect of fluid exsolution on Fe isotopes.The ?56Fe do not show a clear correlation with ?56 Zn and radiogenic isotopes,suggesting that thermal diffusion or crustal contamination cannot produce the high ?56Fe in Hailar volcanic rocks.Fe isotopic variation in Hailar volcanic rocks can be explained by two steps of magmatism.During the first step,partial melting of basaltic trachyandesites with average ?56Fe of 0.09±0.14‰ produced trachytes-rhyodacites with an average ?56 Fe of 0.24±0.27‰.Modelling using rhyolite-MELTS shows that Fe isotopes can be fractionated by preferential partitioning of isotopically different Fe3+ and Fe2+ between the solid residue and partial melt.The second step involves formation of rhyolite with significantly high ?56 Fe through partial melting or extensive crystallization of crust materials,during which isotopically heavy Fe preferentially partition into the rhyolitic melt.Therefore,fractionation of Fe isotopes between melts and minerals can result in high ?56 Fe in SiO2-rich igneous rocks and apparent Fe isotope heterogeneity within the continental crust.To explore the potential of Cu isotopes as a tracer of hydrothermal-fluid activity,copper isotope fractionation between Cl-bearing fluids and silicate magmas(andesite,dacite,rhyolite dacite,rhyolite,and haplogranite)were experimentally calibrated.Fluids containing 1.75 to 14 wt%Cl were welded together with rock powders in Au95Cu5 alloy capsules,which were equilibrated in cold-seal pressure vessels for 7 to 13 days at 800 to 850 ? and 2 kbar.The elemental and Cu isotopic compositions of the recovered fluid and solid phases were analyzed by(LA-)ICP-MS and MC-ICP-MS,respectively.Our experimental results show that the fluid phases are consistently enriched in heavy Cu isotope(65Cu)relative to coexisting silicates,with isotope fractionation factor(A65CuFLUID-MELT)ranging from 0.09%‰ to 0.69‰.Within the experimental conditions in this study,the magnitude of Cu isotopic fractionation between fluid and silicates primarily depends on the Cu speciation in the fluids and silicate melts.At fixed Cl concentration(3.5 wt%)and temperature(850?),A65Cu FLUID-MELT is correlated with fluid constituents caused by varying composition of starting rocks,indicating the influence of fluid components on Cu speciation in fluids.As temperature and salinity can also affect the composition of fluid,these factors exert comprehensive influences on Cu isotope fractionation between fluids and melt.Our results indicate that the exsolved fluids may have higher ?65Cu than the residue magmas,which can be used as a monitor for volatile fluxing.Our data also have implications in explanation the Cu isotope anomaly of the rocks that have reacted with hydrothermal fluids.Together with previous studies on Cu isotope in the brine and vapor phases of porphyry deposits,the results in this study may be utilized for better understanding copper mineralization.